Spacecraft short term pointing stabilitiy

SOHO COMMISSIONING PHASE TEST SHEET

1. TEST IDENTIFICATION
   Short-Term Pointing Stability Test.

2. TEST PURPOSE
   The purpose of this test is to determine the short-term pointing stability (including jitter) of the MDI image during spacecraft and instrument disturbances. The MDI instrument will make image stabilization system (ISS) measurements during mechanism operation of spacecraft reaction wheels and several instruments: CDS, SUMER and UVCS.

3. APPLICABLE PHASE
   Cruise.

4. SCHEDULED DAY
   These tests should be performed sufficiently early in the cruise phase to allow time for further testing and development of an operational response if serious jitter problems are detected. MDI can be quite flexible in scheduling the tests. The participating instruments must have completed necessary system commissioning; for MDI this means completion of ISS Commissioning Tests. The spacecraft pointing system should have been optimized to close to the nominal Halo Orbit stability.

   It is not necessary to test all instruments on the same occasion; MDI can collect the measurements whenever another instrument operates its disturbing mechanisms. The reaction wheels should be measured when they pass through known resonance frequencies of the spacecraft, as identified by analysis or the microvibration test.

5. TYPE OF MEASUREMENT
   The MDI instrument will observe the sun while in "ISS Diagnostic" Mode. In this mode the image motion at the MDI prime focus is measured by limb sensor photodiodes, and the image motion (error) and PZT (actuator) signals are digitized at a 512 Hz sample rate. The digitizied ISS data are sent in the MDI 160k telemetry channel.

6. SUCCESS CRITERIA AND/OR EXPECTED OUTPUT
   The expected output is an off-line determination of the pointing perturbation resulting from specific operations of selected instruments. For the reaction wheels, jitter at the wheel fundamental frequency and its harmonics will be measured.

7. METHOD / DURATION
   A series of disturbance measurements are performed for selected instruments with a duration of approximately 60 minutes each. If all the instrument disturbance measurements are performed at one time, it is expected that the entire set could be completed in a typical 8 hour contact.

8. PROCEDURE REFERENCE
   
   • Setup MDI for jitter mode test (flight version of jtinit)
   • Background observation of "quiet" spacecraft - no disturbances
   • MDI NRT commands to adjust ISS as necessary during the test
   • Begin CDS specific disturbances
   • Begin SUMER specific disturbances
   • Begin UVCS specific disturbances
   • Begin combined instrument disturbances
   • Setup MDI for nominal observing

9. CONSTRAINTS / PREREQUISITES
   The MDI high rate telemetry (VC2 or VC3) is required during the entire test. During periods of planned disturbances, the VC2 telemetry mode is preferred in order to have near-real time data at the EOF in order to verify that the test is being monitored correctly. During periods when VC2 telemetry is impractical, the spacecraft must be in VC3 telemetry mode in order to recover the MDI measurements.

   In order to provide flexibility in the test, NRT commanding is preferred for the instrument specific operations.

10. SPECIFIC REQUIREMENTS
    

    ---

    Rock Bush
    Last modification: June 2, 1995